What is the difference between allerga and allerga d?

Decongestant. Allegra d has the same antihistamine plus an added decongestant. This extra med can help open up swollen nasal passages, and can be used if needed from time to time. However, the decongestant can increased the heart rate and may increase blood pressure, among other things. It should be avoided in someone with heart disease and should not be taken long term. See you doctor for more info.